Layered heaters are typically used in applications where space is limited, when heat output needs vary across a surface, where rapid thermal response is desirous, or in ultra-clean applications where moisture or other contaminants can migrate into conventional heaters. A layered heater generally comprises layers of different materials, namely, a dielectric and a resistive material, which are applied to a substrate. The dielectric material is applied first to the substrate and provides electrical isolation between the substrate and the electrically-live resistive material and also minimizes current leakage to ground during operation. The resistive material is applied to the dielectric material in a predetermined pattern and provides a resistive heater circuit. The layered heater also includes leads that connect the resistive heater circuit to an electrical power source, which is typically cycled by a temperature controller and an over-mold material that protects the lead-to-resistive circuit interface. This lead-to-resistive circuit interface is also typically protected both mechanically and electrically from extraneous contact by providing strain relief and electrical isolation through a protective layer. Accordingly, layered heaters are highly customizable for a variety of heating applications.
Layered heaters may be “thick” film, “thin” film, or “thermally sprayed,” among others, wherein the primary difference between these types of layered heaters is the method in which the layers are formed. For example, the layers for thick film heaters are typically formed using processes such as screen printing, decal application, or film printing heads, among others. The layers for thin film heaters are typically formed using deposition processes such as ion plating, sputtering, chemical vapor deposition (CVD), and physical vapor deposition (PVD), among others. Yet another series of processes distinct from thin and thick film techniques are those known as thermal spraying processes, which may include by way of example flame spraying, plasma spraying, wire arc spraying, and HVOF (High Velocity Oxygen Fuel), among others.
With thick film layered heaters, the type of material that may be used as the substrate is limited due to the incompatibility of the thick film layered processes with certain substrate materials. For example, 304 stainless steel for high temperature applications is without a compatible thick film dielectric material due to the relatively high coefficient of thermal expansion of the stainless steel substrate. The thick film dielectric materials that will adhere to this stainless steel are most typically limited in temperature that the system can endure before (a) the dielectric becomes unacceptably “conductive” or (b) the dielectric delaminates or suffers some other sort of performance degradation. Additionally, the processes for thick film layered heaters involve multiple drying and high temperature firing steps for each coat within each of the dielectric, resistive element, and protective layers. As a result, processing of a thick film layered heater involves multiple processing sequences.
Similar limitations exist for other layered heaters using the processes of thin film and thermal spraying. For example, if a resistive layer is formed using a thermal spraying process, the pattern of the resistive element must be formed by a subsequent operation such as laser etching or water-jet carving, unless a process such as shadow masking is employed, which often results in imperfect resistor patterns. As a result, two separate process steps are required to form the resistive layer pattern. Therefore, each of the processes used for layered heaters has inherent drawbacks and inefficiencies compared with other processes.